1. Field of the Invention
This invention relates to the field of devices that read and write information from and to a magnetic medium, and in particular to the testing of the magnetic heads within a drive device.
2. Description of the Prior Art
Magnetic recording devices are used to write information to, or read information from, a magnetic medium, such as a floppy disc or a hard disc for storing programs and data in a computer.
The read channel for a magnetic recording device includes a sensor in the form of a magneto-resistive head in close proximity with the magnetic medium. When the magnetic material is moved relative to the sensor, a flux is induced in the sensor in dependence upon the local orientation of the magnetic material, thereby generating an information signal that can be amplified and then decoded.
When information is written in digital form, such as for computer data storage or digital recording of music, a current is generated by a write channel and passed through a thin film head in one direction to write a binary xe2x80x9c0xe2x80x9d and in the opposite direction to write a xe2x80x9c1xe2x80x9d. When the medium is read by the sensor, or read head, the portions recorded with a binary xe2x80x9c0xe2x80x9d will induce a current in the head in the one direction and portions recorded with a binary xe2x80x9c1xe2x80x9d will induce a current in the opposite direction, which is then decoded by a bit detector.
To achieve high density or high speed, or both, multiple heads are often used. A hard disk drive, for example, typically includes multiple magnetic discs, or platters, each side of which is used for reading and writing information. The read heads and write heads are mounted on movable arms positioned at each side of the disk. FIG. 1 illustrates an example block diagram of a xe2x80x9cpre-ampxe2x80x9d 100 for a disk drive with multiple read heads 121a-121n and multiple write heads 141a-141n. This pre-amp 100 is typically controlled by a microcontroller 50 in the disk drive that processes commands from a higher level system (not shown) for reading, writing, and configuring the disk drive. Each head 121a-n, 141a-n is uniquely addressable by a head selector 110, in combination with a signal (R/Wxe2x80x2) that indicates whether a read head 121a-n or a write head 141a-n is being accessed. Typically, the head selector selects one of the heads H based on an address provided by the microcontroller 50 on the address bus 102. Each head has an associated xe2x80x9cReader Front Endxe2x80x9d (RFE) 120. The RFE 120 contains an amplifier 125 that provides the read signal from the selected read head 121 to a xe2x80x9cReader Back Endxe2x80x9d (RBE) 130. The RBE 130 further processes the read signal, via filters, amplifiers, and the like 132 to reduce noise and other anomalies that are associated with the response of a read head 121 to the magnetic flux on a magnetic medium (not shown) in the vicinity of the read head 121. As illustrated, due to the relatively low signal to noise ratio that is common in disk drives, differential signaling is used. The buffer 135 provides the differential read signals RDp and RDn to a read/write channel interface device 190 that converts this differential input into a logic value corresponding to the information that is encoded as flux on the magnetic medium in the vicinity of the select read head 121. This logic information is communicated to a microcontroller 50, typically as a collection of logic values forming a byte or word. The communicated information may be, for example, the information bits of a database file, a text or graphics file, an audio or video recording, and so on. In a complimentary manner, information that is to be written to the magnetic medium in the vicinity of the select write head 141 is provided as differential write signals WDp and WDn that are processed by a xe2x80x9cWrite Front Endxe2x80x9d (WFE) 150 and provided to the xe2x80x9cWrite Part Per Headxe2x80x9d (WPPH) 140 corresponding to the selected write head 141. The WFE 150 includes an input buffer 155, filters, amplifiers, and the like 152, whose primary function is to convert the different voltage inputs WDp and WDn from the channel interface 190 into a differential current output that is provided to the write amplifier 145 of the selected WPPH 140.
Illustrated in FIG. 1 is an MR bias setting block 160 that is configured to control the amount of bias current that is applied to each read head 121. Bias current is typically provided to magneto-resistive (MR) heads to improve noise immunity and linearity. Also illustrated in FIG. 1 is a current source 170 that is configured to provide the relatively high (0.1 amps) current that is used for driving the thin-film (TF) heads to produce the required magnitude of flux on the magnetic medium. Both the bias setting block 160 and the write current source 170 are programmable by the microcontroller 50 to provide dynamically adjustable currents, to compensate for manufacturing variations in the performance of the read 121 and write 141 heads. Not illustrated, but common in the art, the pre-amp 100 also includes circuitry that is used to test and characterize the performance of the read 121 and write 141 heads so that the microcontroller 50 can effect the required compensation.
One of the most critical steps in the assembly of a disk drive is the sorting and connecting of the read 121a-n and write 141a-n heads to the preamplifiers 125 and drivers 145 that are used to sense or drive the heads. This is often a yield limiting operation, and costly to repair. Disk drive manufacturers require that preamplifiers contain test capabilities that can be used to determine whether each head is connected to the preamplifier, before final assembly, and as required after final assembly. To remain competitive, however, the cost of adding such capabilities to a preamplifier must be kept to a minimum, and the test circuitry must not degrade the operational performance of the preamplifier. Because the test mode is available after the disk drive is placed in operational use, the test mode must be configured to assure that existing data on the storage medium is not corrupted during the test.
It is an object of this invention to provide a cost effective method and system for testing magnetic heads in a disk drive. It is a further object of this invention to provide a method and system for determining the number of magnetic heads that are connected in a disk drive (the xe2x80x9cform factorxe2x80x9d of the disk drive) without adversely affecting the performance of the operational circuitry.
These objects and others are achieved by providing a circuit that tests the connectivity of each magnetic head in a disk drive by driving each head with a small current, and detecting the flow of this current. By driving and sensing the current flowing through the magnetic heads, both open-circuit faults and bridging faults can be detected. In a preferred embodiment, each head is tested sequentially. The result of each test is stored as a bit value in a register, for subsequent access. The circuit may be activated by a test device, or by a microcontroller in an assembled disk drive. To minimize costs, the circuit is integral to the circuitry that is conventionally used to read and write information via the magnetic heads. The oscillator that is conventionally used to characterize the read heads of a disk drive is used in a preferred embodiment to control the sequencing of tests through each head. A very low common mode voltage is provided during the testing of each write head, to minimize ESD (electro-static discharge) related problems. In a test mode, each write head is selected after a read head is selected, thereby minimizing the possibility of an accidental corruption of the data when the write head is tested. To avoid a degradation of performance, and in particular a degradation of rise/fall times in the write driver, the circuitry that controls the test voltages and currents is provided substantially up-stream of the write driver.